The present invention relates to a pressure sensor in which a pressure sensor chip made of α-phase aluminum oxide (alumina) such as sapphire is joined to a plate member also made of α-alumina such as sapphire, and a method for manufacturing the pressure sensor.
A capacitive pressure sensor chip is available, which detects a pressure by detecting an electrostatic capacitance. This pressure sensor chip has a housing comprised of a base with a predetermined space and a diaphragm arranged above the space of the base, a stationary electrode arranged in the base, and a movable electrode fixed to the diaphragm. In this chip, when the diaphragm deforms by receiving a pressure, a gap between the movable and stationary electrodes changes, thereby changing the electrostatic capacitance therebetween. A pressure received by the diaphragm can be measured on the basis of the change in electrostatic capacitance.
A pressure sensor chip having a housing comprised of a base and diaphragm made of sapphire is proposed. Sapphire, i.e., a corundum (a single crystal of α-phase aluminum oxide) has high thermal stability and can hardly dissolve in an acid and alkaline solution, so that it is widely used as a refractory material, insulator, abrasive, or the like. By using sapphire with the properties described above for the housing, even when an object to be measured is a corrosive fluid, the above-described pressure sensor chip can measure the pressure of the fluid by directly receiving it at the diaphragm.
The above-described pressure sensor chip is used as a pressure sensor by fixing to a pedestal, as shown in FIG. 10. As shown in FIG. 10, a pressure sensor chip 1001 is placed in a recess 1003 formed at a central portion of a pedestal 1002 made of, e.g., glass to face a diaphragm portion upward in FIG. 10. Terminals 1005 of electrode pins 1004 extending through the bottom surface of the pedestal 1002 are formed on the bottom surface of the recess 1003 of the pedestal 1002, which are respectively connected to the wires of electrodes led to the lower surface of the base of the chip 1001. A vent port 1006 communicating the bottom surface of the pedestal 1002 is formed in the bottom surface of the recess 1003.
A cover plate 1007 having an opening portion at the central portion presses the periphery of the upper surface of the chip 1001, thereby fixing it to the recess 1003. The cover plate 1007 is joined to the upper surface of the pedestal 1002 by using glass melted once and fixed to the pedestal 1002. The cover plate 1007 and chip 1001 are airtightly sealed with each other such that no fluid to be measured brought into contact with the upper surface of the chip 1001 enters the recess 1003 around the chip 1001.
To ensure pressure measurement accuracy, the cover plate 1007 is made of sapphire which is the same material as that of the chip 1001 to obtain the same degree of deformation, due to temperatures, as that of the chip 1001, such that no stress acts on the chip 1001. To airtightly seal the chip as described above by excluding dissimilar metal joint as much as possible, the abutting surfaces of the cover plate 1007 and chip 1001 are directly joined. To join the two members made of sapphire as described above, their joint surfaces are mirror-polished and abutted, and the two materials made of sapphire to be joined are heated while applying a pressure to a portion therebetween, thereby joining them strongly without using an adhesive or the like.
In the above-described direct joining, however, since the abutting surfaces of the two materials made of sapphire must be mirror-polished to a roughness of 0.3 nm or less, the expensive member (cover plate) is required. The above-described direct joining without an adhesive or the like generally increases the cost. In contrast, when a general adhesive is used, the two members are joined at a lower cost, thereby manufacturing an inexpensive pressure sensor. However, since the general adhesive is a different material interposed between the two members, a stress occurs, and the same corrosion resistance and thermal stability as in sapphire are not obtained at the joint portion. This restricts the applicable range of the pressure sensor.
As described above, to take full advantage of the material properties of the housing of the pressure sensor, direct joining is conventionally performed at a high cost between the respective members constituting the housing. In contrast, when the respective members are joined by using an adhesive or the like, the component is formed at a low cost, but the full advantage of the material properties of the member is not taken.